The present invention pertains to optical exposure apparatus and methods, and in particular to such apparatus and methods wherein the optical materials comprising the apparatus are susceptible to changes in their optical properties due to the presence of matter, such as moisture and organic compounds, in the atmosphere surrounding the apparatus.
Present-day manufacturing of semiconductor devices, which includes integrated circuits, liquid crystal displays, thin film magnetic heads, and the like, employs optical exposure apparatus and methods. The increasing degree of integration of such semiconductor devices has placed increasing demands on the optical exposure apparatus to achieve higher levels of resolution. The resolution of an optical exposure apparatus can be approximated by the relation
R=kxc3x97xcex/NAP
wherein, R is the xe2x80x9cresolutionxe2x80x9d or resolving power of the optical exposure apparatus (i.e., the size of the smallest feature that can be printed), xcex is the wavelength of the exposure light, NAP is the numerical aperture of the projection lens included therein, and k is a constant which depends upon on the type of recording medium used and the process for developing the images formed therein.
To keep up with the increasing degree of integration of semiconductor devices, continuing efforts are being made, as can be understood from the above formula, to increase the resolution of the optical exposure apparatus by shortening the wavelength of the exposure light and/or increasing NAP. In recent years, KrF (krypton fluoride) excimer lasers having an output wavelength of 248 nm have been used for the exposure light source. Moreover, projection lenses NAP of 0.6 or greater have been commercialized, and features as small as 0.25 microns have been realized.
More recently, in an effort to increase resolution ArF (argon fluoride) excimer lasers having an output wavelength of 193 nm have been gaining attention as a successor light source to KrF excimer lasers. This reduction in wavelength could, in principle, allow the printing of features 0.18 microns or less. However, optical exposure apparatus operable at deep ultraviolet (xe2x80x9cDUVxe2x80x9d, i.e., less than 200 nm) wavelengths are difficult to realize. One reason for this is that in this wavelength region, the materials available for the necessary optical components are currently limited to quartz and calcium fluoride (fluorite). For these materials to be suitable for use in DUV optical exposure apparatuses, they must have sufficient transmittance and internal uniformity (an internal transmittance of 0.995/cm or greater has been achieved with fused quartz, and negligible levels of absorption have been achieved with calcium fluorite). Also, optical components made from these materials require an anti-reflection coating on their surfaces when used at DUV wavelengths, to increase light transmission.
However, even with anti-reflection coatings and minimum levels of absorption, the optical characteristics of fused quartz and calcium fluorite can change due to the heat generated by surface contaminants, which absorb DUV light. Also, moisture and organic substances in the air easily adhere to the lens surfaces of lenses used in the optical exposure apparatuses discussed above. The lens surfaces can be contaminated by these contaminants during the manufacturing of the optical exposure apparatus, as well as during its maintenance. In particular, since these contaminants strongly absorb light having a wavelength of less than 200 nm, transmittance in optical exposure apparatus that use exposure light of less than 200 nm is reduced due to such contaminants adhering to the lens surfaces. For instance, it has been discovered that the transmittance of optical components made from fused quartz and calcium fluoride drops rapidly when exposed to moisture or organic compounds. The amount of this absorption, which can reach up to 0.01 per lens surface, is large compared to the absorption due to the material itself or the surface anti-reflection coatings. Therefore, it is necessary to keep the surfaces of fused quartz or calcium fluorite optical components free of such contaminants.
Japanese patent application Kokai No. Hei 7-294705 discloses a technique relating to a method for photo-cleaning individual optical components with light (hereinafter, xe2x80x9cphotocleaningxe2x80x9d). In the photo-cleaning technique disclosed therein, the contaminants adhering to the lens surfaces, in the manner described above, separate from the lens surfaces when irradiated with ultraviolet light, effectively cleaning the lens surfaces. Further, when exposure light of less than 200 nm used in the optical exposure apparatus is ultraviolet light, the contaminants adhering to the lens surfaces may be photo-cleaned by operating the optical exposure apparatus and irradiating the lenses of the optical system with the exposure light. However, this technique does not disclose a method for photo-cleaning all, or the essential optical parts of, the optical components of an optical exposure apparatus after the apparatus has been assembled. It has been discovered by the present inventors that the temporary photo-cleaning of individual optical components by exposing them to DUV light actually facilitates the later absorption of ambient moisture and organic compounds onto the surfaces of the optical components. Consequently, even if individual optical components are photo-cleaned using DUV light, it is extremely difficult to assemble the optical components to form a projection exposure system, and then isolate those components completely from moisture, organic compounds, and other contaminants. This has been a major impediment in realizing a robust DUV projection exposure system.
Nevertheless, the numerical aperture NAI of illumination optical systems used in projection exposure apparatuses are generally smaller than NAP of projection lens. Consequently, only one part of the NA region of a projection lens (i.e., the region corresponding to NAI) is illuminated if the illumination light from the illumination optical system impinges directly onto the projection lens. The result is that only the illuminated region is photo-cleaned. This is problematic because cleaned regions having high transmittance and contaminant-adhered regions having low transmittance arise on the lens surfaces, resulting in unevenness (i.e., non-uniformity) in the amount of light in the mask pattern image. This unevenness is due to transmittance unevenness and degradation of the resolving power caused by a reduction in the effective NA of the projection lens, which causes a drop in the imaging performance of the projection exposure apparatus. This has been another major impediment in realizing a robust DUV projection exposure system.
The present invention pertains to optical exposure apparatus and methods, and in particular to such apparatus and methods wherein the optical materials comprising the apparatus are susceptible to changes in their optical properties due to the presence of organic matter, such as moisture and organic compounds, in the atmosphere surrounding the apparatus.
More particularly, a first aspect of the invention is an optical exposure apparatus for forming an image on a photosensitive substrate. The apparatus comprises an illumination optical system having a light source and a light beam. Next to the illumination optical system is a reticle having a pattern. Next to the reticle on the opposite side of the illumination optical system is a projection lens. The projection lens and the illumination optical system have a predetermined space therein. An exposure optical path (i.e., the light path associated with performing an exposure) passes through this predetermined space. An optical path deflection member is removably arranged in the predetermined space so as to cause a deflection in the exposure optical path to form a second optical path. The second optical path differs from the exposure optical path. The second optical path is the light path associated with performing photo-cleaning. The optical path deflection member can be, for example, a rotating prism or one or more moving optical components (including lenses and mirrors).
In a second aspect of the invention, instead of an optical path deflection member, a light diffusing member is placed in the light beam associated with performing an exposure (i.e., the exposure light beam) so as to create a second light beam that is larger than the exposure light beam. This second light beam is the light beam associated with performing photo-cleaning.
In a third aspect of the invention, the optical path deflection member or light diffusing member is removably disposed within the predetermined space so that photo-cleaning can be performed between exposures.
A fourth aspect of the invention is a method of photo-cleaning an optical exposure apparatus for forming an image on a photosensitive substrate. The method comprises the steps of first, forming an exposure optical path in a predetermined space within an illumination optical system and a projection lens. The second step is preventing the photosensitive substrate from being in said exposure optical path. The third step is changing the exposure optical path to an optical path that differs from said exposure optical path.
A fifth aspect of the invention is a method of photo-cleaning a projection lens having an object plane. The method comprises a first step of providing an illumination light beam along an optical axis. The second step is providing and inserting a photo-cleaning optical member having refractive power into the illumination light beam. The third and last step is refracting the illumination light beam with the photo-cleaning optical member so as to illuminate lens surfaces of one or more lenses included in the projection lens.
A sixth aspect of the invention is an optical exposure apparatus capable of imaging a pattern present on a mask onto a photosensitive substrate. The apparatus comprises an illumination optical system with a light source for generating a light beam. A projection lens having an object plane is disposed adjacent the illumination optical system. A photo-cleaning optical member is removably provided between the illumination optical system and the projection lens and within the light beam so as to refract the light beam to impinge of the projection lens.